英文:
Cast nested List<List<T>> to nested ArrayList<ArrayList<T>>
问题
什么是将 List<List<T>> 强制转换为 ArrayList<ArrayList<T>> 的最佳/性能最佳的方法？
假设我有一个名为 IFoo.getListOfLists() 的接口，它将给我 List<List<T>>。同时假设我知道这些 List<List<T>> 实际上是 ArrayList<ArrayList<T>> 的实例。
一个简单的解决方案是：
ArrayList<ArrayList<T>> result = new ArrayList<>();
for (List<T> arrayList : IFoo.getListOfLists()) {
result.add((ArrayList<T>)arrayList);
}
但是是否有更好的方法呢？尤其是因为我已经知道这些实例实际上是 ArrayList。
英文:
What's the best / most performant way to cast a List<List<T>> to ArrayList<ArrayList<T>>?
Let's assume I have an interface IFoo.getListOfLists() which will give me the List<List<T>>. Let's also assume I know that these List<List<T>> are in fact instances of ArrayList<ArrayList<T>>.
One simple solution would be:
ArrayList<ArrayList<T>> result = new ArrayList<>();
for (List<T> arrayList : IFoo.getListOfLists()) {
result.add((ArrayList<T>)arrayList);
}
but is there a better approach? Especially, because I already know that the instances are in fact ArrayLists.
答案1
得分: 1
由于在运行时缺少对List的类型信息，您可以简单地将其转换为ArrayList：
import java.util.ArrayList;
import java.util.List;
public class CastListList {
public static void main(final String[] args) {
final List<List<String>> strings = new ArrayList<>();
strings.add(new ArrayList<>());
strings.get(0).add("Hello World!");
final ArrayList<ArrayList<String>> arrayList = cast(strings);
System.out.println(arrayList.get(0).get(0));
}
@SuppressWarnings({ "unchecked", "rawtypes" })
private static <T> ArrayList<ArrayList<T>> cast(final List<List<T>> list) {
return (ArrayList) list;
}
}
英文:
since the type information for the List is absent at runtime, you can simply cast to ArrayList:
import java.util.ArrayList;
import java.util.List;
public class CastListList {
public static void main(final String[] args) {
final List<List<String>> strings = new ArrayList<>();
strings.add(new ArrayList<>());
strings.get(0).add("Hello World!");
final ArrayList<ArrayList<String>> arrayList = cast(strings);
System.out.println(arrayList.get(0).get(0));
}
@SuppressWarnings({ "unchecked", "rawtypes" })
private static <T> ArrayList<ArrayList<T>> cast(final List<List<T>> list) {
return (ArrayList) list;
}
}
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